PROJECT SUMMARY Our long-term research goals are to understand the mechanisms that regulate stem cell fate decisions. Here, we propose to pursue questions in fetal hematopoiesis by investigating the role, regulation and specification of a novel population of hematopoietic stem cells (HSC). Although this population fulfills the most stringent criteria for functional HSC, their life-span during normal development is restricted to a limited developmental window. A functional HSC that does not persist into adulthood has never been observed before and therefore defines a novel wave of definitive hematopoiesis with a distinct endpoint. Our discovery of these novel HSC provides a unique opportunity to pursue the mechanisms that regulate the persistence of stem cells throughout life. In addition, the existence of stage-specific HSC will provide insights to how specialized immune cells are established during specific developmental timepoints to meet the varying demands on the immune system. Because our finding is based on an irreversible genetic deletion event, our model is also uniquely suited for understanding the relationship between cell populations during HSC specification and differentiation during development. These questions will be pursued by a combination of transplantation assays and in vitro experiments. The outcomes will provide essential new insights on the establishment of the blood and immune system guide strategies for combatting developmental hematopoietic disorders, including certain types of anemia, autoimmune disease, and childhood cancer. .